Research on men who have sex with men (MSM) in Africa has been limited, although this population is particularly vulnerable to HIV infection. In a study published in the June edition of AIDS, researchers estimate that at least 739 MSM are selling sex to men in and around the city of Mombasa, Kenya – a “sizeable population who urgently need to be targeted by HIV prevention strategies.”
Research and interventions in Africa have largely focused on the heterosexual spread of HIV. Men who have sex with men (MSM) are highly stigmatised and hard to reach. However, it is known that there are male sex workers, including those who sell sex to other men. An ongoing cohort study is investigating MSM who sell sex in and around Mombasa. In this related report, public health researchers from Kenya, Nairobi and elsewhere estimated the total size of this population using a “capture-recapture” methodology.
Thirty-seven MSM “peer leaders” were selected and trained to serve as enumerators for this study. On two consecutive Saturdays in 2006, these enumerators frequented 77 areas in and around Mombasa District known to be key areas for MSM sex trade (bars, nightclubs, private establishments, beaches and parks). Enumerators approached other suspected MSM to establish trust, confirm MSM status, and then determine if the contact was “on the market”, i.e., willing to sell sex. If so, the enumerators identified themselves, then “tagged” the contact by giving them a distinctive leaflet and asked if they had already received the same leaflet from anyone else.
This tagging was used to determine how many MSM “on the market” were contacted or “captured” at each time point, and how many were “recaptured” on both consecutive Saturdays. Any men tagged more than once on the same day were counted only once. After removing these duplicates, a total of 284 men were identified in the first capture and 484 in the second; of these 484, 186 were “recaptures” from the first day.
Briefly, the capture-recapture methodology assumes that the probability of being captured is the same on both days. As a consequence of this assumption, the ratio of first captures to the total population equals the ratio of recaptures to second captures (i.e., if 5% of all MSM who sell sex are captured on the first day, then 5% of those first captures will be recaptured on the second day). By this formula, the estimated total number of MSM who sell sex in this area is (284 x 484 / 186), or 739 (95% confidence interval, 690-798).
Nearly all assumptions and other possible sources of error result in this number likely underestimating the true population. The significantly larger number of captures on the second day is noteworthy: the researchers speculate that this was likely due to improved enumerator skills and confidence, coupled with the fact that the second count was done following a Friday when many Kenyans receive their pay. The report therefore concludes that the “estimate of 739 men who sell sex in and around Mombasa is likely to be conservative.”
In an accompanying editorial, Frits van Griensven of the US Centers for Disease Control states, “the importance of this study does not so much pertain to the accuracy of the estimate, but more to the fact that the investigators were able to identify and enrol hundreds of MSM selling sex in the African country of Kenya.” This likely also points to a much larger MSM population overall, as “most MSM do not sell sex.” Both the study report and editorial stress that MSM in Africa represent a sizeable, highly stigmatised and vulnerable group for whom prevention strategies are urgently needed.